Electro-static discharge protection is a critical factor in circuit design. Therefore, it is critical to provide a discharge path for such discharge events, in order to protect the remainder of the electronic device.
Electro-static discharge (ESD) is a major reliability issue in integrated circuits (ICs). Due to the complexities in RF-ESD co-design, the ESD rating for RF circuits has been traditionally low. Hence there is a growing demand for improvement in the ESD robustness of RF circuits.
The standard ESD circuit is merely a diode. ESD circuits may also be comprised of alternative techniques such as thick field oxide (TFO) clamps, Silicon controlled rectifiers (SCRs), grounded gate NMOS (GGNMOS), spark gaps, or transistors.
Gallium-Arsenide (GaAs) has historically been the preferred technology for Power Amplifiers (PA), Low Noise Amplifiers (LNA) and switches because of its intrinsically higher electron mobility, low base resistance, low-loss semi-insulating substrate, linearity, higher transition frequency and breakdown voltage.
ESD protection of RF circuits designed in GaAs HBT (Heterojunction Bipolar Transistor) and pHEMT (pseudomophic HEMT) technologies is still in its infancy and is the topic of significant research and development. Some novel ESD protection schemes for RF circuits use GaAs processes; however they are not suitable for high power RF ports.
A high electron mobility transistor (HEMT), heterostructure FET (HFET), or modulation-doped FET (MODFET), is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFET. A commonly used material combination is GaAs with AlGaAs, though there is wide variation, dependent on the application of the device. Devices incorporating more Indium generally show better high-frequency performance, while in recent years, gallium nitride HEMTs have attracted attention due to their high-power performance.
To allow conduction, semiconductors are doped with impurities which donate mobile electrons (or holes). However, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. HEMTs avoid this through the use of high mobility electrons generated using the heterojunction of a highly-doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case).
The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a quantum well (a steep canyon) in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low resistivity (or to put it another way, “high electron mobility”). This layer is called a two-dimensional electron gas. As with all the other types of FETs, a voltage applied to the gate alters the conductivity of this layer.
Ideally, the two different materials used for a heterojunction would have the same lattice constant (spacing between the atoms). In practice, e.g. AlGaAs on GaAs, the lattice constants are typically slightly different, resulting in crystal defects. As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form deep-level traps, and greatly reduce device performance. A HEMT where this rule is violated is called a pHEMT or pseudomorphic HEMT. This is achieved by using an extremely thin layer of one of the materials—so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger bandgap differences than otherwise possible, giving them better performance.
Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the mHEMT or metamorphic HEMT, an advancement of the pHEMT. The buffer layer is made of AlInAs, with the Indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain).
The tradeoff for this high performance is a low voltage rating at which catastrophic failure occurs, such as the voltages generated by an electro-static discharge event. Thus, the challenge is to design an electro-static discharge protection circuit structure using HEMTs that can be integrated onto the die.